This invention relates generally to well bore measurement tools and more particularly, but not by way of limitation, to a fracture orientation caliper tool mounted between two interlockable packers.
In fracturing a formation intersected by a well bore, it is known that two seals, referred to as packers, are set in the well at the upper and lower boundaries of the formation to be fractured. A pressurized fracturing fluid is then injected between the set packers through a tubing or pipe string on which the packers are carried into the hole. Such procedure can be used either when the well bore is lined with a casing or when the well bore is unlined (referred to herein as an open well bore, or the like). It is important that once the packers are set, they remain set (until specifically released) so that the fracturing fluid will be properly contained to achieve the desired fracturing and so that hazardous conditions are not thereby created. It is also important for the packers to remain set when a measuring tool, such as a precision caliper tool subassembly, is carried between them. Any movement of the packers that is communicated to a tool such as a caliper could produce false readings and seriously damage such a tool when it has its measurement arms extended.
When the well bore is lined with a casing or the like, known types of mechanical and hydraulic slips can be used to engage the casing so that upward movement of the top packer, such as in response to the pressure of the fracturing fluid exceeding the hydrostatic pressure existing above the top packer, is prevented. Preventing such upward movement can also sometimes be accomplished to some degree by "slacking off" the tubing or pipe string so that the weight of the string exerts a downward acting force on the packers.
When packers are to be set in open well bores, however, the aforementioned mechanical and hydraulic slips have not been helpful in anchoring the top packer against upward movement. Likewise, the use of "slacked-off" tubing has been inadequate in general because in deep wells where the slacked-off pipe weight would be sufficient to prevent upward movement, the weight has been known to create a force exceeding the loading characteristic of the packer, thereby damaging it. In shallower wells, the upward applied force exerted by the fracturing fluid can easily overcome the lesser pipe weight, thereby causing the top packer to become unseated.
The foregoing problem particularly pertains to upward movement of the top packer because the lower packer has the greater fracturing fluid pressure acting downwardly on it, and its downward movement is limited by an anchor pipe testing on the bottom of the hole or engaging the side wall of the bore. The interconnecting construction of conventional dual packers known to the art is such that this downward limitation is also applicable to the top packer so that it is only the upward movement of the upper packer which is of primary concern.
Although one can circumvent this problem by always casing or lining the well bore and by then using the known types of casing engaging locks, it is desirable to solve the problem in a manner whereby open hole packers can be securely set and locked in open well bores because this saves the time and expense of always having to case or line the well bore while still accomplishing reliable fracturing.
The foregoing exemplifies the particular need for a lock by which a top or upper packer can be locked relative to a bottom or lower packer to prevent upward movement of the upper packer in response to the fracturing fluid pressure exerted between the two packers when the packers are used in an open well bore. The satisfaction of this need, however, would also provide an improved lock useful in other types of downhole apparatus which require locking against relative movement between different parts of the apparatus.
There is also the need for a device which can be used with the interlockable packers to determine the direction of a fracture that is created by a hydraulic fracturing process or treatment which is applied between the interlocked packers. This determination can be made with an instrument which measures the deformation of the well bore during the hydraulic pressurization of a fluid contained between the two packers set in the well bore. Such an instrument is referred to as a caliper, of which there are various types known to the art, but for which there is the need for an improved type having several features.
One of the desired features is a construction by which, after being run into the well bore beween the two packers, the caliper can be locked into the formation by its own force applied through a single set of implements which securely fasten the caliper to the formation and which also provide movements sensitive to the deformation of the formation. The mounting of this device between the packers should be in such a manner that if either or both of the packers moves relative to the formation, such movement does not affect the operation of the caliper.
The forces which are to be applied through the single set of implements are preferably generated by a single drive unit to simplify the construction and maintenance of the caliper. Such single drive unit preferably, however, is capable of applying independent forces to the individual holding implements to accommodate the various dispositions of the caliper in the well bore, which has a side wall that will never be perfectly round and thus never evenly spaced from the caliper. Such single drive unit must be capable of creating forces great enough to securely fix the caliper to the formation.
To further maintain simplicity of design, the implements used to secure the caliper to the formation should be the same ones to detect deflections of the formation caused by the fracturing process. Such detections should be highly sensitive so that accurate in situ stress measurements, which are fundamental to understanding rock fracture mechanics, can be obtained. In addition to the taking of such highly sensitive measurements, however, the tool should also be capable of making relatively large measurements from which the total radial movement of the arms can be measured to determine diameters or transverse dimensions of the hole in which the tool is used.
Such an improved caliper tool should also be capable of measuring the forces applied to the formation engaging implements so that other properties of the formation can be determined. For example, rock hardness can be determined knowing the forces applied to the formation through the implements and the distances the implements have moved.
Despite having such a novel combination of features as just described, the tool should also be able to maintain features found in existing tools, such as means for measuring pressure, temperature and orientation.